UTRAN (Universal Terrestrial Radio Access Network) is a term that identifies the radio access network of a UMTS (Universal Mobile Telecommunications System), wherein the UTRAN consists of Radio Network Controllers (RNCs) and NodeBs i.e. radio base stations. The NodeBs communicate wirelessly with mobile user equipments (UEs) and the RNCs control the NodeBs. The RNCs are further connected to the Core Network (CN). Evolved UTRAN (E-UTRAN) is an evolution of the UTRAN towards a high-data rate, low-latency and packet-optimised radio access network. Further, the E-UTRAN consists of e-NodeBs (evolved NodeBs), and the e-NodeBs are interconnected and further connected to the Evolved Packet Core network (EPC). E-UTRAN is also being referred to as Long Term Evolution (LTE) and is standardized within the 3rd Generation Partnership Project (3GPP).
In a time multiplexed system, e.g. the uplink in E-UTRAN, HSPA (High Speed Packet Access) or GSM (Global System for Mobile communications) the transmitters transmit in certain assigned timeslots. Thus, a transmitter will start transmitting in the beginning of the timeslot and turn off the transmitter at the end of the timeslot. In addition it is possible that the output power of the transmitter may change from timeslot to timeslot or within a timeslot.
Transmitters typically require some time for turning on the output power as well as turning off the output power. This means that the turning on and off the output power does not occur instantaneously. Furthermore, very sharp transitions between on state and off state would cause unwanted signal emissions in the adjacent carriers causing adjacent channel interference, which should be limited to certain level. Thus, there exists a transient period, i.e. when the transmitter switches from the off state to the on state or vice versa. During these transient periods the output signal of the transmitter is undefined in the sense that the quality of the signal is not as good as when the transmitter is fully turned on. The transient periods are illustrated in FIG. 1. Furthermore, the output power during the transient period is referred to as a power ramp.
As illustrated in FIG. 1, the duration of the ramping is typically quite short compared to the length of the sub-frame or timeslot but its position has an influence on system performance. In terms of ramping or transient position there are three possibilities:                Ramping outside the timeslot/sub-frame as illustrated in FIG. 2a         Ramping inside the timeslot/sub-frame as illustrated in FIG. 2b         Ramping partly inside and partly outside the timeslot/sub-frame as illustrated in FIG. 2c         
A power mask, also referred to as a time mask, defines for example the allowed output power at given time instants during a transient event and the time when a ramp starts. For example when the transmitter ramps up, i.e. increases the output power, the power mask may specify how much output power is allowed before the transient event, during the transient event and after the transient event and additionally, when the ramp-up should start. The allowed output power may be expressed as an open range, i.e. below a specific level or as an interval, i.e. between output power X and Y.
It should be noted that in GSM and WCDMA (Wideband Code Division Multiple Access) the power masks are defined in timeslot level (577 μs and 667 μs respectively). In E-UTRAN it will be defined on sub-frame level (1 ms) and SC-OFDM (Single Carrier-Orthogonal Frequency-Division Multiplexing) symbol level, e.g. to be applied when a Sounding Reference Symbol (SRS) is transmitted in the sub-frame.
There are several methods currently in use for avoiding the adverse effects of the ramping periods. In GSM and UTRA-TDD (Universal Terrestrial Radio Access-Time-Division Duplex) the transmitter is turned on slightly before the actual signal is transmitted. In that way the transmitter has some time to reach the on state before the actual signal is transmitted. At the end of the timeslot the transmitter is not turned off until the complete signal has been transmitted. If timeslots are adjacent in time and energy is transmitted outside the timeslot the transmitted energy from one user equipment will cause interference to the signal from another user equipment. To mitigate this problem a tiny guard interval is introduced between the timeslots. In UTRA-FDD (UTRA-Frequency-Division Duplex) this solution is not utilized. The transmitter has not fully reached the on-state when the signal is transmitted and the transmitter is turned off before the transmission of the signal has been completed. In this case the coding and spreading of the signal will mitigate the effects of the ramping period.
In the UTRAN the power control operates on timeslot level. This means that power change occurs on timeslot basis and the transmit power mask is consequently defined on timeslot basis. Moreover, in E-UTRAN the power control operates on sub-frame basis and therefore the transmit power mask is defined on sub-frame level and OFDM symbol level.
As mentioned previously, in E-UTRA uplink the duration of a sub-frame is 1 ms. The sub-frame consists of 14 or 12 SC-OFDM symbols. The last symbol in the sub-frame could be used for transmitting the SRS that is used for channel estimation purposes. The SRS can also be used for performing uplink channel dependent scheduling and time tracking. The transmit power for the SRS may differ from the transmit power used for the other symbols of the sub-frame The relationship of the different transmit powers is illustrated in FIG. 3. However, it should be noted that the abrupt power changes shown in FIG. 3 are not possible to implement.
In the E-UTRAN the uplink timeslots are placed adjacent to each other in time. In the state of the art solution that exists for UTRA, one set of fixed well defined ramped up and down periods are defined in the standard 3GPP TS 25.101 and TS 25.102. Thus, the tradeoff between signal quality and interference to other timeslots is set when the system is designed. FIG. 4 illustrates that placement of power ramps causes problems with signal quality degradation due to non-constant output power and with interference to a user. However, certain signals, e.g. the sounding reference symbol (SRS), need to have good quality especially when they are utilized for uplink channel dependent scheduling. Furthermore, in other situations the interference due to power ramping needs to be minimized in respect to other signals such as data symbols in order to maximize throughput.
Accordingly, there is a need for an improved transmission output power control in the E-UTRAN.